Controlling gene expression in transgenics

Over-expression of a NAC 67 transcription factor from finger millet (Eleusine coracana L.) confers tolerance against salinity and drought stress in rice

Background

NAC proteins (NAM (No apical meristem), ATAF (Arabidopsis transcription activation factor) and CUC (cup-shaped cotyledon)) are plant-specific transcription factors reported to be involved in regulating growth, development and stress responses. Salinity responsive transcriptome profiling in a set of contrasting finger millet genotypes through RNA-sequencing resulted in the identification of a NAC homolog (EcNAC 67) exhibiting differential salinity responsive expression pattern.

Methods

Full length cDNA of EcNAC67 was isolated, characterized and validated for its role in abiotic stress tolerance through agrobacterium mediated genetic transformation in a rice cultivar ASD16.

Results

Bioinformatics analysis of putative NAC transcription factor (TF) isolated from a salinity tolerant finger millet showed its genetic relatedness to NAC67 family TFs in related cereals. Putative transgenic lines of rice over-expressing EcNAC67 were generated through Agrobacterium mediated transformation and presence/integration of transgene was confirmed through PCR and southern hybridization analysis. Transgenic rice plants harboring EcNAC67 showed enhanced tolerance against drought and salinity under greenhouse conditions. Transgenic rice plants were found to possess higher root and shoot biomass during stress and showed better revival ability upon relief from salinity stress. Upon drought stress, transgenic lines were found to maintain higher relative water content and lesser reduction in grain yield when compared to non-transgenic ASD16 plants. Drought induced spikelet sterility was found to be much lower in the transgenic lines than the non-transgenic ASD16.

Conclusion

Results revealed the significant role of EcNAC67 in modulating responses against dehydration stress in rice. No detectable abnormalities in the phenotypic traits were observed in the transgenic plants under normal growth conditions. Results indicate that EcNAC67 can be used as a novel source for engineering tolerance against drought and salinity stress in rice and other crop plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0261-1) contains supplementary material, which is available to authorized users.

SOS1 gene overexpression increased salt tolerance in transgenic tobacco by maintaining a higher K(+)/Na(+) ratio

Crop productivity is greatly affected by soil salinity, so improvement in salinity tolerance of crops is a major objective of many studies. We overexpressed the Arabidopsis thaliana SOS1 gene, which encodes a plasma membrane Na(+)/H(+) antiporter, in tobacco (Nicotiana tabacum cv. Xanthi-nc). Compared with nontransgenic plants, seeds from transgenic tobacco had better germination under 120 mM (mmol L(-1)) NaCl stress; chlorophyll loss in the transgenic seedlings treated with 360 mM NaCl was less; transgenic tobacco showed superior growth after irrigation with NaCl solutions; and transgenic seedlings with 150 mM NaCl stress accumulated less Na(+) and more K(+). In addition, roots of SOS1-overexpressing seedlings lost less K(+) instantaneously in response to 50 mM NaCl than control plants. These results showed that the A. thaliana SOS1 gene potentially can improve the salt tolerance of other plant species.

WUS and STM-based reporter genes for studying meristem development in poplar

We describe the development of a reporter system for monitoring meristem initiation in poplar using promoters of poplar homologs to the meristem-active regulatory genes WUSCHEL (WUS) and SHOOTMERISTEMLESS (STM). When ~3 kb of the 5' flanking regions of close homologs were used to drive expression of the GUSPlus gene, 50-60% of the transgenic events showed expression in apical and axillary meristems. However, expression was also common in other organs, including in leaf veins (40 and 46% of WUS and STM transgenic events, respectively) and hydathodes (56% of WUS transgenic events). Histochemical GUS staining of explants during callogenesis and shoot regeneration using in vitro stems as explants showed that expression was detectable prior to visible shoot development, starting 3-15 days after explants were placed onto callus inducing medium. A minority of WUS and STM events also showed expression in the cambium, phloem, or xylem of regenerated, greenhouse grown plants undergoing secondary growth. Based on microarray gene expression data, a paralog of poplar WUS was detectably up-regulated during shoot initiation, but the other paralog was not. Both paralogs of poplar STM were down-regulated threefold to sixfold during early callus initiation. We identified 15-35 copies of cytokinin response regulator binding motifs (ARR1AT) and one copy of the auxin response element (AuxRE) in both promoters. Several of the events recovered may be useful for studying the process of primary and secondary meristem development, including treatments intended to stimulate meristem development to promote clonal propagation and genetic transformation.

Heterologous expression of glycosyl hydrolases in planta: a new departure for biofuels

The concept of expressing non-plant glycosyl hydrolase genes in plant tissue is nearly two decades old, yet relatively little work in this field has been reported. However, resurgent interest in technologies aimed at enabling processes that convert biomass to sugars and fuels has turned attention toward this intuitive solution. There are several challenges facing researchers in this field, including the development of better and more specifically targeted delivery systems for hydrolytic genes, the successful folding and post-translational modification of heterologous proteins and the development of cost-effective process strategies utilizing these transformed plants. The integration of these concepts, from the improvement of biomass production and conversion characteristics to the heterologous production of glycosyl hydrolases in a high yielding bioenergy crop, holds considerable promise for improving the lignocellulosic conversion of biomass to ethanol and subsequently to fuels.

Intron-hairpin RNA derived from replication associated protein C1 gene confers immunity to tomato yellow leaf curl virus infection in transgenic tomato plants

The whitefly-transmitted Tomato Yellow Leaf Curl Virus (TYLCV) is the major pathogen of tomato crop in Cuba and one of the most outstanding viral diseases of plants worldwide. In this work, we have developed transgenic tomato plants, transformed with an intron-hairpin genetic construction to induce post- transcriptional gene silencing against the early TYLCV replication associated protein gene (C1). The intron-hairpin RNA produced involves 726 nts of the 3' end of the TYLCV C1 gene as the arms of the hairpin, and the castor bean catalase intron. Transgenic tomato plants belonging to line 126, which harbor a single transgene copy, showed immunity to TYLCV, even in extreme conditions of infection (4-leaf-stage plants and 300 to many hundreds viruliferous whiteflies per plant during 60 days). Dot blot hybridization of these plants showed no TYLCV DNA presence 60 days after inoculation. Small interfering RNA molecules were detected in both inoculated and non-inoculated plants from line 126. These transgenic tomato plants of the otherwise very TYLCV-susceptible Campbell-28 tomato cultivar, are the first report of resistance to a plant DNA virus obtained by the use of the intron-hairpin RNA approach.

High accumulation of bioactive peptide in transgenic rice seeds by expression of introduced multiple genes

We have developed a simple binary vector construction system for the simultaneous expression of multiple genes in plants. Up to three independent gene cassettes can be easily integrated into one binary vector using the MultiSite Gateway System. Using this system, we produced transgenic rice plants that accumulated high levels of the hypocholesterolaemic peptide lactostatin (IIAEK) in endosperm. Binary vectors were constructed that could accommodate up to three independent modified glutelin gene cassettes encoding multimer lactostatin in the variable regions. Eight construct permutations were used for rice transformation. We measured the accumulation of lactostatin expressed as a glutelin fusion protein in the mature seeds of 105 independent transgenic rice lines. A general correlation was observed between accumulation level and gene number in the vector constructs, indicating that a higher accumulation of lactostatin was obtained from transgenic rice plants containing the maximum number of gene inserts. These results indicate that this strategy is applicable for the selection of transgenic lines containing large amounts of bioactive peptides in rice seeds.

Optimization of Acidothermus cellulolyticus endoglucanase (E1) production in transgenic tobacco plants by transcriptional, post-transcription and post-translational modification

An attempt was made to obtain a high-level production of intact Acidothermus cellulolyticus endoglucanase (E1) in transgenic tobacco plants. The E1 expression was examined under the control of the constitutive and strong Mac promoter or light-inducible tomato Rubisco small sub-unit (RbcS-3C) promoter with its original or Alfalfa Mosaic Virus (AMV) RNA4 5'-untranslated leader (UTL) and targeted to different sub-cellular compartments via transit peptides. The transit peptides included native E1, endoplasmic reticulum, vacuole, apoplast, and chloroplast. E1 expression and its stability in transgenic plants were determined via E1 activity, protein immunoblotting, and RNA gel-blotting analyses. Effects of sub-cellular compartments on E1 production and its stability were determined in transgenic tobacco plants carrying one of six transgene expression vectors, where the E1 was under the control of Mac promoter, mannopine synthase transcription terminator, and one of the five transit peptides. Transgenic tobacco plants with an apoplastic transit peptide had the highest average E1 activity and protein accumulation, which was about 0.25% of total leaf soluble proteins estimated via E1 specific activity and protein gel blots. Intercellular fluid analyses confirmed that E1 signal peptide functioned properly in tobacco cells to secret E1 protein into the apoplast. By replacing RbcS-3C UTL with AMV RNA4 UTL E1 production was enhanced more than twofold, while it was less effective than the mannopine synthase UTL. It was observed that RbcS-3C promoter was more favorable for E1 expression in transgenic plants than the Mac promoter. E1 activity in dried tobacco seeds stored one year at room temperature was 45% higher than that observed immediately after harvesting, suggesting that E1 protein can be stored at room temperature for a long period. E1 stability in different sub-cellular compartments and the optimal combination of promoter, 5'-UTL, and sub-cellular compartmentation for heterologous protein production in transgenic plants are discussed.

Transient expression analysis of allelic variants of a VNTR in the dopamine transporter gene (DAT1)

Background

The 10-repeat allele of a variable number tandem repeat (VNTR) polymorphism in the 3'-untranslated region of the dopamine transporter gene (DAT1) has been associated with a range of psychiatric phenotypes, most notably attention-deficit hyperactivity disorder. The mechanism for this association is not yet understood, although several lines of evidence implicate variation in gene expression. In this study we have characterised the genomic structure of the 9- and 10-repeat VNTR alleles, and directly examined the role of the polymorphism in mediating gene expression by measuring comparative in vitro cellular expression using a reporter-gene assay system.

Results

Differences in the sequence of the 9- and 10- repeat alleles were confirmed but no polymorphic differences were observed between individuals. There was no difference in expression of reporter gene constructs containing the two alleles.

Conclusions

Our data suggests that this VNTR polymorphism may not have a direct effect on DAT1 expression and that the associations observed with psychiatric phenotypes may be mediated via linkage disequilibrium with other functional polymorphisms.

A short synthetic MAR positively affects transgene expression in rice and Arabidopsis

Matrix Attachment Regions (MARs) are DNA elements that are thought to influence gene expression by anchoring active chromatin domains to the nuclear matrix. When flanking a construct in transgenic plants, MARs could be useful for enhancing transgene expression. Naturally occurring MARs have a number of sequence features and DNA elements in common, and using different subsets of these sequence elements, three independent synthetic MARs were created. Although short, these MARs were able to bind nuclear scaffold preparations with an affinity equal to or greater than naturally occurring plant MARs. One synthetic MAR was extensively tested for its effect on transgene expression, using different MAR orientations, plant promoters, transformation methods and plant species. This MAR was able to increase average transgene expression and produced integration patterns of lower complexity. These data show the potential of making well defined synthetic MARs and using them to improve transgene expression.

Improved in planta expression of the human islet autoantigen glutamic acid decarboxylase (GAD65)

The smaller isoform of the enzyme glutamic acid decarboxylase (GAD65) is a major islet autoantigen in autoimmune type 1 diabetes mellitus (T1DM). Transgenic plants expressing human GAD65 (hGAD65) are a potential means of direct oral administration of the islet autoantigen in order to induce tolerance and prevent clinical onset of disease. We have previously reported the successful generation of transgenic tobacco and carrot that express immunoreactive, full-length hGAD65. In the present study, we tested the hypothesis that the expression levels of recombinant hGAD65 in transgenic plants can be increased by targeting the enzyme to the plant cell cytosol and by mediating expression through the potato virus X (PVX) vector. By substituting the NH2-terminal region of hGAD65 with a homologous region of rat GAD67, a chimeric GAD67(1-87)/GAD65(88-585) molecule was expressed in transgenic tobacco plants. Immunolocalization analysis showed that immunoreactive GAD67/65 was found in the plant cell cytosol. By using a radio-immuno assay with human serum from a GAD65 autoantibody-positive T1DM patient, the highest expression level of the recombinant GAD67/65 protein was estimated to be 0.19% of the total soluble protein, compared to only 0.04% of wild-type hGAD65. Transient expression of wild-type, full-length hGAD65 in N. benthamiana mediated by PVX infection was associated with expression levels of immunoreactive protein as high as 2.2% of total soluble protein. This substantial improvement of the expression of hGAD65 in plants paves the way for immunoprevention studies of oral administration of GAD65-containing transgenic plant material in animal models of spontaneous autoimmune diabetes.

Molecular evolution and genetic engineering of C4 photosynthetic enzymes

The majority of terrestrial plants, including many important crops such as rice, wheat, soybean, and potato, are classified as C(3) plants that assimilate atmospheric CO(2) directly through the C(3) photosynthetic pathway. C(4) plants, such as maize and sugarcane, evolved from C(3) plants, acquiring the C(4) photosynthetic pathway in addition to the C(3) pathway to achieve high photosynthetic performance and high water- and nitrogen-use efficiencies. Consequently, the transfer of C(4) traits to C(3) plants is one strategy being adopted for improving the photosynthetic performance of C(3) plants. The recent application of recombinant DNA technology has made considerable progress in the molecular engineering of photosynthetic genes in the past ten years. It has deepened understanding of the evolutionary scenario of the C(4) photosynthetic genes. The strategy, based on the evolutionary scenario, has enabled enzymes involved in the C(4) pathway to be expressed at high levels and in desired locations in the leaves of C(3) plants. Although overproduction of a single C(4) enzyme can alter the carbon metabolism of C(3) plants, it does not show any positive effects on photosynthesis. Transgenic C(3) plants overproducing multiple enzymes are now being produced for improving the photosynthetic performance of C(3) plants.

Controlling gene expression in plants using synthetic zinc finger transcription factors

Synthetic zinc finger proteins can be fused to transcriptional regulatory domains to create artificial transcription factors that modulate the expression of a specific target gene. Recent studies have demonstrated that synthetic zinc finger domains can be constructed to bind DNA sequences with a high degree of specificity. To devise a general strategy for controlling plant gene expression with artificial transcription factors, a rapid transient assay was developed to test the regulatory activity of synthetic zinc finger transcription factors (effectors) on target plasmids (reporters) in plant cells. Effective activation was demonstrated with zinc finger proteins fused to a derivative of the VP16 activation domain. The mSin3 interaction domain (SID) of the human MAD1 protein provided moderate repression of target reporters. Unlike many naturally occurring transcription factors, these synthetic effectors exhibit a strong dependence on binding site position. Reporter genes that are stably integrated into plant cells responded similarly to transiently transfected reporter plasmids, verifying that this assay accurately reflects the behavior of these transcription factors on an endogenous target within the context of chromosomal DNA. These results provide evidence that synthetic zinc finger proteins can be used to manipulate the expression of endogenous genes in plants.

Expressing creatine kinase in transgenic tobacco--a first step towards introducing an energy buffering system in plants

Creatine kinase a key enzyme in cellular energy homeostasis of vertebrates offers the promise of engineering plants with enhanced stress tolerance. In order to provide plants with such an energy buffering system, tobacco was transformed with a cDNA, encoding the cytosolic brain-type isoform of chicken creatine kinase (BB-CK), the expression of which was under the control of the cauliflower mosaic virus 35S (CaMV 35S) promoter. Transgenic tobacco plants were selected and suspension cultures generated. Both transgenic plants and suspension cultures were shown to stably express enzymatically active BB-CK in vitro and in vivo, and in most cases for three successive generations (T0-T2). Exogenously supplied creatine was shown to enter the plant cells and resulted in only a slight reduction in root growth at concentrations up to 10 mM. Furthermore, the BB-CK expressing tobacco plants and cell suspension cultures were able to convert creatine into phosphocreatine.

Manipulating gene expression for the metabolic engineering of plants

Introducing and expressing foreign genes in plants present many technical challenges that are not encountered with microbial systems. This review addresses the variety of issues that must be considered and the variety of options that are available, in terms of choosing transformation systems and designing recombinant transgenes to ensure appropriate expression in plant cells. Tissue specificity and proper developmental regulation, as well as proper subcellular localization of products, must be dealt with for successful metabolic engineering in plants..

Selection and orientation of adjacent genes influences DAM-mediated male sterility in transformed maize

Anther-targeted expression of E. coli DNA (Adenosine-N6-)-Methyltransferase (DAM) in maize was tested as a means to produce male-sterile plants. A high frequency of male-sterile plants with reduced anther size was observed when DAM was regulated by the maize anther-specific promoter 5126 (5126:DAM) and placed upstream of the herbicide resistance gene, pat, regulated by the cauliflower mosaic virus (CaMV) 35S promoter (35S:PAT). In contrast, placement of 5126:DAM upstream of a pat gene regulated by either the maize ubiquitin (UBI:PAT) or rice actin (rACTIN:PAT) promoters resulted in male-fertile plants. Based on these observed differences, DAM-mediated sterility was used as a phenotypic marker to assess the contribution of factors affecting gene expression such as orientation of the transcription units, choice of regulatory sequences mediating expression of adjacent genes, and effects of varying the anther-specific promoter regulating DAM. Constructs that place a portion of the CaMV 35S promoter, including the native AS-1 sequences, between 5126:DAM and UBI:PAT yielded a high frequency of male-sterile plants with reduced anther size. Significant differences in the frequency of male-sterile events and the associated anther size were also observed when the position of 35S:PAT was changed relative to 5126:DAM. These data provide evidence that gene expression in transformed maize plants can be impacted by simply altering the order, orientation or regulatory sequences of adjacent genes.

Molecular farming of pharmaceutical proteins

Molecular farming is the production of pharmaceutically important and commercially valuable proteins in plants. Its purpose is to provide a safe and inexpensive means for the mass production of recombinant pharmaceutical proteins. Complex mammalian proteins can be produced in transformed plants or transformed plant suspension cells. Plants are suitable for the production of pharmaceutical proteins on a field scale because the expressed proteins are functional and almost indistinguishable from their mammalian counterparts. The breadth of therapeutic proteins produced by plants range from interleukins to recombinant antibodies. Molecular farming in plants has the potential to provide virtually unlimited quantities of recombinant proteins for use as diagnostic and therapeutic tools in health care and the life sciences. Plants produce a large amount of biomass and protein production can be increased using plant suspension cell culture in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can also produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the molecular farming of recombinant therapeutics, including vaccines, diagnostics, such as recombinant antibodies, plasma proteins, cytokines and growth factors.

Shunting is a translation strategy used by plant pararetroviruses (Caulimoviridae)

In eukaryotes standard initiation of translation involved 40S ribosome scanning to bridge the distance from the cap to the initiation codon. Recently deviations from that rule had been described, including "internal initiation", "poly-A dependent translation", and "ribosome shunting". In ribosome shunting, ribosomes start scanning at the cap but large portions of the leader are skipped. Thereby the secondary structure of the shunted region is preserved. Scanning in plant caulimoviruses involve a small open reading frame properly spaced in front of a strong stem structure, and, in order to function, the small open reading frome has to be translated and the peptide released. This arrangement can be mimicked by artificial small open reading frames and stem structures. Shunting with viral and synthetic leaders occurs not only in plant-, but also in mammalian and yeast systems. Thus it responds to an intrinsic property of the eukaryotic translational machinery and probably acts in many cases where coding regions are preceded by complex leaders.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.

Molecular farming of recombinant antibodies in plants

'Molecular farming' is the production of recombinant proteins in plants. It is intended to harness the power of agriculture to cultivate and harvest transgenic plants producing recombinant therapeutics. Molecular farming has the potential to provide virtually unlimited quantities of recombinant antibodies for use as diagnostic and therapeutic tools in both health care and the life sciences. Importantly, recombinant antibody expression can be used to modify the inherent properties of plants, for example by using expressed antipathogen antibodies to increase disease resistance. Plant transformation is technically straightforward for model plant species and some cereals, and the functional expression of recombinant proteins can be rapidly analyzed using transient expression systems in intact or virally infected plants. Protein production can then be increased using plant suspension cell production in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can be exploited to produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the 'molecular farming' of recombinant therapeutics, blood substitutes and diagnostics, such as recombinant antibodies.